Control systems for liquid desiccant air conditioning systems

ABSTRACT

Methods and control systems are disclosed for operating a liquid desiccant air-conditioning system to efficiently maintain a target temperature and humidity level in a space.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication No. 62/580,249 filed on Nov. 1, 2017 entitled CONTROLSYSTEMS FOR LIQUID DESICCANT AIR CONDITIONING SYSTEMS, which is herebyincorporated by reference.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention under ContractNo. DE-AC36-08G028308 between the United States Department of Energy andthe Alliance for Sustainable Energy, LLC, the Manager and Operator ofthe National Renewable Energy Laboratory.

BACKGROUND

The present application relates generally to the use of liquiddesiccants (LD) in combination with heat pumps, compressors, andchillers to condition the temperature and humidity of an air streamentering a space. Current control systems are designed for DirecteXpansion (DX) systems or solid desiccant wheel systems. Liquiddesiccant air conditioning systems (LDAC) allow for significantindependent control of humidity and temperature, while reducing theenergy required to achieve specific supply air targets by up to 50%. Thecapability to Independently control of temperature and humidity suppliedby LDAC systems not only improves comfort and health, but alsosimplifies building management controls and reduces the risk of humiditydamage to the building. Depending on the latent and sensible loads thatneed to be managed, the system can either heat and simultaneouslyhumidify air, or heat and simultaneously dehumidify or cool the airwhile humidifying or dehumidifying the air. This enables system managersto maintain more comfortable and healthier indoor air conditions thanconventional systems can provide. Such independent control of humidityand temperature is critical for many applications, including but notlimited to outside air supply to commercial buildings in monsoon regionsor the use of air conditioners in buildings which have spaces with verydifferent sensible heat ratio (SHR) requirements, like grocery storeswith high humidity and relatively warm grocery/bakery sections and dryand cool refrigeration sections. Controlling systems and buildings tomeet such load characteristics requires appropriate control strategiesfor individual pieces of equipment and for the complete buildings.

Desiccant dehumidification systems—both liquid and solid desiccants—havebeen used in parallel to conventional vapor compression HVAC equipmentto help reduce humidity in spaces, particularly in spaces that requirelarge amounts of outdoor air or that have large humidity loads insidethe building space itself. (ASHRAE 2012 Handbook of HVAC Systems andEquipment, Chapter 24, p. 24.10). Humid climates, such as Miami, Fla.,require a lot of energy to properly treat (dehumidify and cool) thefresh air that is required for a space's occupant comfort. Soliddesiccant dehumidification systems have been used for many years and aregenerally quite efficient at removing moisture from the air stream.However, liquid desiccant systems generally use concentrated saltsolutions such as ionic solutions of LiCl, LiBr, or CaCl2 and water.Membrane based liquid desiccant systems have been primarily applied tounitary rooftop units for commercial buildings. However, in addition torooftop units, commercial buildings also use air handlers located insidetechnical spaces in the building for the cooling and heating of bothoutside air and recirculated air. There is an additional substantialmarket for chillers that provide cold water to coils inside the buildingand use evaporative cooling for high efficiency condensers. Residentialand small commercial buildings often use split air conditioners whereinthe condenser (together with the compressor and control system) islocated outside and one or more evaporator cooling coils is installed inthe space than needs to be cooled. In Asia in particular (which isgenerally hot and humid) the split air conditioning system is thepreferred method of cooling (and sometimes heating) a space. Each ofthose require different configurations and control mechanisms. Variousconfigurations for managing humidity and temperature independently havebeen disclosed in U.S. Pat. No. 9,243,810 and U.S. Patent ApplicationNo. 62/580270. They disclose configurations with liquid desiccantcomponents, sensible coils, direct and indirect evaporative cooling andwater addition. They disclose how performance for various modes ofoperation is optimized, including cooling and dehumidification, coolingonly, cooling and humidification, heating only, heating andhumidification and heating and dehumidification. Cooling and heatingrefer here to changing the DB condition only; changes in total enthalpywill be described as net heating and net cooling. Existing controlstrategies need to be adjusted to optimize efficiency and comfort ineach of these operating modes for the different configurations.

Liquid desiccant systems generally have two separate functions. Theconditioning side of the system provides conditioning of air to therequired conditions, which are typically set using thermostats orhumidistats. The regeneration side of the system provides areconditioning function of the liquid desiccant so that it can bere-used on the conditioning side. Liquid desiccant is typically pumpedor moved between the two sides, and a control system helps to ensurethat the flows and concentrations of the liquid desiccant is properlybalanced between the two sides as conditions necessitate and that excessheat and moisture are properly dealt with, without leading toover-concentrating or under-concentrating of the desiccant.

Performance of the conditioner and regenerator is driven by the flowrates and temperatures the three fluids: air, water, and desiccant inthe heat exchangers. The dehumidification potential is driven by theconcentration of the desiccant, which can be controlled in a number ofways as disclosed, e.g., in U.S. Pat. No. 9,243,810 and U.S. PatentApplication No. 62/580270. The primary controls are compressor power andfan speed and water flow of the regenerator. Adding sensible coolingcapacity with additional air-to-water or air-to-refrigerant coils cangreatly broaden the amount work done by the system and the range ofsensible heat ratios that can be supported by increasing the ability forindependent control of temperature and humidity within the unit.

At a building level, overall effectiveness of the system is driven bythe mix of systems used and how they are used. For example, the ASHRAEDOAS (Dedicated Outside Air Unit) design guide identifies howconditioning the outside air to handle the complete dehumidificationload of the building can increase overall efficiency, due to the muchimproved efficiency of air cooled coils when only used for sensiblecooling,.

The benefits of liquid desiccant systems have been described in variouspatents, e.g., U.S. Pat. No. 9,243,810 and others. Such systems havebeen clearly demonstrated for hot and humid climates with a large latentload. As buildings get better insulated, these latent loads increase asa percentage of total cooling loads, making effective dehumidificationmore important. As internal sensible loads are reduced in tighter,better insulated buildings, conditioning ventilation air becomes an evenmore significant part of total cooling and heating loads.

Extreme design conditions, including very humid and cool, very hot anddry, and very humid and cold require special cooling and heatingsolutions for which earlier liquid desiccant systems are not optimized.

At very high temperatures (>100 F) and very low humidity (<20% RH),liquid desiccant systems can't operate efficiently and need specialcontrols to avoid crystallization of the desiccant. Traditionalevaporative cooling systems do well at low humidities and moderatecooling requirements, but are unable to deal with extreme heat or withmore humid conditions that tend to occur at least part of the time inmost locations.

Traditional cooling systems use refrigerant coils that are air cooledand are best suited for sensible cooling. Condense forming on the coilacts as an insulator that reduces its capacity. Thus multiple coils needto be used in series to fully dehumidify and cool the air. Four and sixrow coils are commonly used. Still, traditional systems often cannothandle the full latent load without significantly overcooling the airand then reheating it, or mixing high volumes of return air with smallvolumes of outside air to minimize the humidity level of the mix.Especially in times where only a small amount of sensible cooling isrequired, humidity control is compromised. When compressors are cycledto manage smaller loads, bursts of humidity enter the building as coilswarm up and evaporate condense back into the air. Many split systemsprovide heating by operating as a reversible heat pump system. Theliquid desiccant system is able to cool outside air without frostforming, significantly improving system efficiency by reducing oreliminating defrost cycles. The removal of humidity from the outside airalso enables the humidification of the heated space, maintaining healthyRH levels between 30-60% RH. These tend to be most useful in moderateclimates where cooling and heating loads are roughly in balance. Verycold climates like the Midwest and Northeast of the US still requireadditional heating, often from natural gas or oil. In more moderateclimates, heat pump effectiveness is limited by humidity, which can leadto frost forming and the use of very inefficient defrost cycles. Using aliquid desiccant condenser coil prevents frost forming in a heat pumpsystem.

Liquid desiccants can achieve effective dehumidification at highertemperatures of the compressors evaporator, during the cooling cycle.The regenerator fully rejects the condenser energy at lower temperaturesthan traditional air cooled systems. As a result, the compressor canmove energy from the conditioned space to outside the space at a muchlower temperature differential than traditional systems. This improvesthe efficiency of the compressor in proportion to the reduction in thetemperature difference. The lower temperature difference between thecondenser and evaporator is the lift of the compressor and drives theefficiency of the combination of compressor-based cooling and heatingwith liquid desiccant heat exchangers.

As disclosed in U.S. Pat. No. 9,243,810, by actively diluting thedesiccant, e.g., by using vapor transfer membrane modules, a liquiddesiccant system can increase the ratio of sensible cooling to latentcooling. It can even starts to act like a direct evaporative cooler,which allows it to maintain a target minimum dewpoint (DP) under verydry conditions, without maintaining dry bulb (DB) targets at highwetbulb (WB) conditions, something traditional direct evaporators cannotdo. While the conditioner operates at cooler temperatures the overalltemperature differential over the compressor tends to be further reduceddue to a much larger reduction in condenser temperatures, as diluteddesiccant increases evaporation at the condenser transforming it into ade facto water cooled air conditioning system with comparable efficiencybut with significantly reduced water consumption.

Existing control strategies for air conditioners can rely on bandwidthcontrol, adaptive control or predictive control, they need to bemodified for the various configurations of liquid desiccant systems andthen optimized for each of the operating modes described before.

Additional building humidity “guidelines” are being developed toencourage maximum, and sometimes even minimum, humidity levels mostlydriven by health considerations, especially the impact on respiratorydisease and allergies.

In dry climates, water cooled chillers and evaporative coolers use theevaporation energy of water to cool spaces and/or improve compressorefficiency, but this uses potable water in substantial volumes. Managingthe scaling effects and biological pollution of such water is asignificant challenge. In locations where both humid and dry conditionsoccur, evaporative chillers are less effective. Standard liquiddesiccant solutions do not operate well under those conditions. We willdisclose how water addition can be controlled while simplifying liquiddesiccant systems, making them competitive in both dry and humidconditions. We will also disclose that using vapor transfer modules inliquid desiccant systems reduces water consumption significantly overthe life time of an installation when compared to traditionalevaporative coolers, a critical consideration in many climate zones.

Many buildings have to deal with a variety of conditions from very hotand dry to relatively cool and humid including high DP/high relativehumidity (RH) and high DB/low DP design points. We will disclose howliquid desiccant systems can be controlled to handle these conditionseffectively. This includes the use of a combined liquid desiccant systemwith direct evaporation of the air supplied to the regenerator andindirect evaporative cooling of supply air after dehumidification. Bothsignificantly improve system performance in dry and hot climates. Theyare identical in their effect on the system to direct dilution of theliquid desiccant by adding demineralized water to the liquid desiccanttank or using membrane modules to transfer water from a feedstream tothe highly concentrated liquid desiccant.

Since the concentration in the liquid desiccant system drives the RH ofboth the supply conditions in the conditioner as well as the outputconditions of the regenerator, liquid desiccant systems operate as“constant RH machines.” This allows for different control strategies,including controls based RH and WB targets and measurements rather thanDB and DP. These can be simpler and more effective than traditionalcontrol in independently controlling humidity and temperature.

Measuring the concentration can be done in a variety of ways. We willdisclose how this can be used for different control strategies, using acombination of RH of the regenerator exhaust and the conditioner supply,tank levels, electrical resistance, defraction, specific weight of thedesiccant, and measuring concentration based on viscosity andtemperature.

Desiccant dilution through vapor transfer or forward osmosis can be donein a variety of ways.

Controlling for crystallization uses empirical data on crystallizationpoints at different temperatures and humidity levels. Preventingcrystallization which stop the system's ability to manage humidity iscritical. And since crystallization only occurs at conditions that aretoo dry for comfort, avoiding those conditions tend to improve supplyconditions that most benefit building occupants.

Energy recovery is a major factor in managing air quality and theefficiency of air conditioning systems. We will disclose how this canimpact control strategies.

Concentrated liquid desiccants are a very efficient form of energystorage with more KwH per lb. than ice. We will disclose how controlstrategies can make optimal use of regeneration capacity due to wasteheat, solar or other heat resources.

Frost control is a critical consideration in any heatpump system. Liquiddesiccant systems can avoid them by proper sizing of system components,improving overall system efficiency. Frost prevention strategies will bedisclosed

Liquid desiccant systems have distinct modes depending on therelationship between input conditions and target conditions. We willdiscuss the main modes, their impact on bandwidth/adaptive systems aswell as predictive systems. Including transition between modes withoutflip flopping.

SUMMARY

In accordance with one or more embodiments, a method is disclosed ofoperating a liquid desiccant air-conditioning system to maintain targettemperature and humidity level in a space. The liquid desiccant airconditioning system comprises: (a) a conditioner for treating a firstair stream flowing therethrough and provided to the space as a suppliedair stream, said conditioner using a heat transfer fluid and a liquiddesiccant to treat the first air stream; (b) a device for measuringtemperature and a device for measuring humidity in the supplied airstream; (c) a regenerator connected to the conditioner such that theliquid desiccant can be circulated between the regenerator and theconditioner, the regenerator causing the liquid desiccant to desorbwater vapor to a second air stream or to absorb water vapor from thesecond air stream depending on a selected mode of operation of thesystem; (d) a refrigerant system; (e) a first refrigerant-to-heattransfer fluid heat exchanger connected to the conditioner and therefrigerant system for exchanging heat between the refrigerant heated orcooled by the refrigerant system and the heat transfer fluid used in theconditioner; (f) a second refrigerant-to-heat transfer fluid heatexchanger connected to the regenerator and the refrigerant system forexchanging heat between the refrigerant heated or cooled by therefrigerant system and the heat transfer fluid used in the regenerator;and (g) a system controller for controlling operation of the system. Themethod comprises the steps of:

-   (i) measuring the temperature and humidity level in the supplied air    stream;-   (ii) comparing the temperature measured in (a) to a target    temperature to determining a temperature error, and comparing the    humidity level measured in (a) to a target humidity level to    determine a humidity error;-   (iii) comparing the humidity error and the temperature error on a    common scale to determine the greater error;-   (iv) using the greater error to drive the system controller to    control operation of the system to reduce the greater error;-   (v) repeat (i) through (iv) a plurality of times.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a simplified diagram illustrating an exemplary 3-way liquiddesiccant air conditioning system using a chiller or external heating orcooling sources.

FIG. 2 illustrates an exemplary single membrane plate in a liquiddesiccant air conditioning system.

FIG. 3 shows the basic design of the membrane plate of FIG. 2.

FIG. 4A shows a basic version of a liquid desiccant air conditioningsystem in cooling mode.

FIG. 4B shows a wide range of options to enhance RH control in a liquiddesiccant system.

FIG. 5 shows the psychrometric chart for the basic DOAS concentrationwith limited independent control of humidity and temperature.

FIG. 6 show an a system with all controls in the refrigerant system incooling mode.

FIG. 7A shows a heat pump system for cooling and heating withrefrigerant coils and one dual coil in cooling mode for hot and dryconditions.

FIG. 7B shows a heat pump system for cooling and heating with watercooled coils and a reversible compressor.

FIG. 8 shows a multi zone building with ceiling units, a DOAS unit andseparate regeneration units.

FIG. 9 shows how in a split system a mid-unit can be used to improvecontrol over pressure and flow in the heat transfer and desiccantchannels for multi-floor and larger buildings.

FIG. 10 shows how in a multi zone building multiple tanks with differentconcentrations of desiccants can be used to optimize controls, includingmanaging spaces with different humidity loads and target supplyconditions, but also for rapid response to changing conditions and forenergy conservations and peak shaving.

FIG. 11 shows multiple options for adding water to liquid desiccantincluding evaporator pads, vapor transfer units, forward osmosis systemsand direct addition of demineralized water to a tank.

FIG. 12 shows a liquid desiccant energy recovery solution.

FIG. 13A shows how different configurations are driven by the change intemperature and humidity between input and target conditions. Eightzones are identified. System configurations are driven by the weathermap for the location and the supply target.

FIG. 13B shows how typical south eastern US conditions are sub-tropicalwith mostly hot and humid during the middle of the day with coolerconditions at night and in the morning. Conditions in the south west ofthe US (e.g., Phoenix) represent a continental climate with conditionsranging from very hot and dry to hot and humid to very cold.

FIG. 13C shows how energy recovery reduces the range of inputconditions, simplifying system requirements.

FIG. 14A shows how the basic configuration of a liquid desiccant systemoperates in different modes.

FIG. 14B shows water addition control settings for the differentoperating zones in the psychrometric chart.

FIG. 14C shows how a second cooled coil linked to the second heatexchanger operates in different operating zones in the psychrometricchart.

FIG. 14D shows how a dual use air cooled coil operates in differentoperating zones in the psychrometric chart.

FIG. 15A shows target supply conditions for a typical air conditioningsystems with significant sensible and latent loads.

FIG. 15B shows how economizer modes in a direct outside air systemdepend on the function of the other components of the HVAC system.

FIG. 16 shows actual performance data of a liquid desiccant system withwater addition.

FIG. 17 shows the main climate zones in the US as well as the annualtemperature and humidity data for major US metropolitan areas.

FIG. 18 shows how different HVAC system reach a given humiditycondition.

FIGS. 19A-19F shows how a system with a single evaporator air cooledcoil can optimize all 4 920 conditions.

FIG. 20 shows how evaporative cooling and water addition can be used tomanage the same conditions.

FIG. 21 shows how indirect or direct evaporative systems cannot matchthe supply conditions of a liquid desiccant system with water additionor combined with direct evaporative cooling.

FIG. 22 gives an overview of three primary heating modes.

FIG. 23A shows where a system without air cooled coils can outperformalternative systems.

FIG. 23B shows how a system with an air cooled “heat dump” coil canoutperform alternative systems.

FIG. 23C shows how a system with water addition and an evaporator aircooled coil can outperform alternative systems.

FIG. 24 shows a psychrometric chart for T&RH sensors and a typicaltarget range

FIG. 25 shows a psychrometric chart for a standard DOAS control system

FIG. 26 shows an advanced liquid desiccant control system to managecapacity and humidity directly

FIG. 27 shows how water addition is not required for a DOAS system thatonly controls for a maximum DP.

FIG. 28 shows a liquid desiccant tank level can be used to controlconcentration and thus RH.

FIG. 29 shows how such an adaptive algorithm can work.

FIG. 30A describes the liquid desiccant air conditioning control logicfor an adaptive system.

FIG. 30B advanced adaptive controls using the largest error in humidityand temperature to from humidity and temperature.

FIG. 31 shows a control logic for the method of FIG. 30.

FIG. 32A shows a basic adaptive control structure using RH and DB.

FIG. 32B provides a more detailed description of the system of FIG. 32A.

FIG. 32C show the inclusion of some predictive controls.

FIG. 33 shows building and system sensors.

FIG. 34A shows the crystallization curve of a desiccant (LiCl).

FIG. 34B shows the implications of the crystallization curve of adesiccant (LiCl) for different operating modes and climates.

FIG. 35 shows an approach to crystallization alarms.

FIG. 36 shows how viscosity changes in a liquid desiccant system betweenconditioner and regenerator.

DETAILED DESCRIPTION

FIG. 1 depicts a new type of liquid desiccant system, as described inmore detail in U.S. Pat. No. 9,243,810, which is incorporated byreference herein. A conditioner 101 comprises a set of plate structuresthat are internally hollow. A cold heat transfer fluid is generated incold source 107 and entered into the plates. Liquid desiccant solutionat 114 runs down the outer surface of each of the plates. The liquiddesiccant runs behind a thin membrane that is located between the airflow and the surface of the plates. Outside air at 103 is blown throughthe set of (wavy) conditioner plates. The liquid desiccant on thesurface of the plates attracts the water vapor in the air flow and thecooling water inside the plates helps to inhibit the air temperaturefrom rising. The treated air at 104 is put into a building space.

The liquid desiccant is collected at the bottom of the wavy conditionerplates at 111 and is transported through a heat exchanger 113, to thetop of the regenerator 102, and to point 115 where the liquid desiccantis distributed across the wavy plates of the regenerator. Return air, oroptionally outside air, at 105 is blown across the regenerator plate andwater vapor is transported from the liquid desiccant into the leavingair stream at 106. An optional heat source 108 provides the drivingforce for the regeneration. The hot transfer fluid at 110 from the heatsource can be put inside the wavy plates of the regenerator similar tothe cold heat transfer fluid on the conditioner. Again, the liquiddesiccant is collected at the bottom of the wavy plates 102 without theneed for either a collection pan or bath, so the regenerator the airflow can be horizontal or vertical. An optional heat pump 116 can beused to provide cooling and heating of the liquid desiccant. It is alsopossible to connect a heat pump between the cold source 107 and the hotsource 108, which is pumping heat from the cooling fluids rather thanthe desiccant.

FIG. 2 describes a 3-way heat exchanger as described in further detailin U.S. Pat. No. 9,308,490, which is incorporated by reference herein. Aliquid desiccant enters the structure through ports 204 and is directedbehind a series of membranes as described in FIG. 1. The liquiddesiccant is collected and removed through ports 205. A cooling orheating fluid is provided through ports 206 and runs counter to the airstream at 201 inside the hollow plate structures, again as described inFIG. 1 and in more detail in FIG. 3. The cooling or heating fluids exitthrough ports 207. The treated air at 202 is directed to a space in abuilding or is exhausted as the case may be. The figure illustrates a3-way heat exchanger in which the air and heat transfer fluid are in aprimarily vertical orientation. It is however also possible to flow theair and the heat transfer fluid in a horizontal aspect, which is notfundamental to the operation of the system.

FIG. 3 describes a 3-way heat exchanger as described in more detail inU.S. Pat. No. 9,631,848, which is incorporated by reference herein. Theair stream at 351 flows counter to a cooling fluid stream at 354.Membranes 352 contain a liquid desiccant at 353 that is flowing alongthe wall 355 that contains a heat transfer fluid at 354. Water vapor at356 entrained in the air stream is able to transition through themembrane 352 and is absorbed into the liquid desiccant at 353. The heatof condensation of water at 358 that is released during the absorptionis conducted through the wall 355 into the heat transfer fluid at 354.Sensible heat at 357 from the air stream is also conducted through themembrane 352, liquid desiccant at 353 and wall 355 into the heattransfer fluid at 354.

FIG. 4A shows a liquid desiccant system with the basic configuration.FIG. 4B shows a system with additional means to regulate humidityindependently from temperature as well for minimizing energyconsumption. Conditioner 416 uses liquid desiccant 425 and hot water 418to process a mixture of outside and return air 403 to supply airconditions at 419. The diluted liquid desiccant 420 is returned to atank and then via a heat exchanger 413 to regenerator 423 where hotwater 440 and a mixture of outside and exhaust air 406 is used toreconcentrate the liquid desiccant 420 to 425, while humidifying andheating exhaust air 406. The cold water 418 and hot water are suppliedby evaporator coil 427 and condenser coil 428 of compressor 417 usingpumps 460 and 461, respectively.

FIG. 4B shows additional coils and water addition options. For example,to reduce the concentration of the regenerated liquid desiccant part ofthe condenser heat can be diverted to air cooled coil 429 using outsideair 430. To reduce the conditioner load, the process air 403 can beprecooled using exhaust air 431 and heat exchanger 432. A second aircooled coil 433 is shown, which can be used to supply some of thecooling load by cooling exhaust air 406 form regenerator 423. Othercoils 434 and 435 can be used to recool or post cool process air. Theconcentration of the liquid desiccant can be reduced using wateraddition modules 436, which can be positioned at different locations inthe liquid desiccant loop 436 a injects demineralized water 437 adirectly into the tank. 436 b uses the heat from the regenerator 423 tomaximize the efficiency of the heat transfer in a vapor unit. 436 c isset up to further dilute the liquid desiccant before entering theregenerator thereby reducing the temperature of the heat transfer fluid440 to condenser coil 428. This reduces lift and thus improves theefficiency of compressor 416. Water injection in 436D gives directcontrol over the RH of the supply air RH 404.

A valve system 441, 442, 443 and 444 is shown in the refrigerantcircuit. For those skilled in the art it will be clear that a similarvalve system in the heat transfer fluid circuit can be used to controlthe air cooled coils

The following fundamental approaches can be used to manage humidity andtemperature independently:

1. Total cooling or heating capacity defined as the change in enthalpyof the system is driven by the capacity of the compressor system 416.

2. The liquid desiccant panels will use the available capacity togenerate air with an RH that is significantly lower than the outside airused for regeneration. Fluid flows 418 and 440 for heat transfer fluidsand 404 and 415 for liquid desiccant through the panels of 417 and 423(air/heat transfer fluid/desiccant) can significantly adjust the ratioof latent versus sensible cooling.

-   3. Adding a sensible coil 429 to the condenser side of the    compressor in parallel or in series to the regenerator 423 enables    additional sensible cooling.-   4. Adding a sensible coil 433 to the evaporator side 427 of the    compressor with outside air or air from the regenerator 423 provides    additional cooling power for deep dehumidification while maintaining    or increasing the air temperature.-   5. Adding a sensible coil 434 before the conditioner on the    evaporator side of the compressor and after the conditioner on the    airside maximizes sensible capacity.-   6. Desiccant dilution 436 allows more sensible cooling. It can be    used for net humidification of supply air and makes it possible to    control humidity in a limited bandwidth.-   7. Preconditioning the air 405 with direct evaporation has an    identical effect to desiccant dilution, but uses existing    components.-   8. Exhaust air 431 can be used on the conditioner 427 to reduce the    overall load, either sensible (Plate heat exchanger (HX)) 432 or    sensible and latent (full enthalpy HX including wheels, plates, and    LD plates).-   9. Exhaust air 431 can be used at the regenerator 423 to provide    deeper dehumidification.

The ability of the system in FIG. 4B to manage temperature and humidityindependently is constrained by the need to balance the compressor.Therefore the cooling power available at the evaporator 427 is alwaysless than the heat available for regeneration at 428. The differenceresults from the heat and friction losses in the compressor. Typicallyregeneration power is 20 to 30% higher than the conditioner power.

FIG. 5 shows the performance of the system in 4A. The psychrometricchart shows how starting from condition 501 a, b and c, the conditionerwill dehumidify and cool to condition 502 a, b, and c. The coolingenergy 510 a (delta enthalpy times airflow needs to balance with theregeneration energy 510 b (delta enthalpy times the regenerationairflow), with the friction/heat losses available as extra regeneratorpower. The concentration at the conditioner and the concentration of LDat the regenerator are in balance with a 1-2% difference which is drivenby the change in concentration within a panel at a given condition. As aresult, the RH 541 at the regenerator and the RH at the conditioner haveto be in balance with corrections for that small difference inconcentration. The humidity absorbed 511 a at the conditioner and thehumidity desorbed 511 b by the regenerator has to be in balance. Theresulting outside air condition, supply conditions and regenerator outconditions are shown in FIG. 5. For condition A, a target condition 502Aat 50% RH and can be reached, but for conditions B and C this may not bepossible Increasing regenerator air and water flows will change supplyconditions for 501C. For example a higher regenerator flow 503 willresult in a lower concentration of the LD and thus a higher supply RH504. The wet bulb condition being the same this will result in a higherDP and lower DB. A lower water flow at the regenerator can have theopposite effect.

For greater flexibility in managing temperature and humidityindependently, additional components can be added to the system (FIG.4B) to improve control over concentration and thus over humidity. Forexample, to achieve a target supply condition similar to 502A from inputcondition 501 c requires a lower concentration of liquid desiccant andmore work. This can be achieved in several ways, among other byrejecting condenser heat through an air cooled coil 429, by directdilution 436 or through the use of an evaporator coil 499 before theregenerator.

FIGS. 6, 7A, and 7B show several ways in which the liquid desiccantconcentration can be changed, including direction and flow of therefrigerant and the heat transfer fluid through valves 617, 618, 619 inFIG. 6 and valves 717, 718, 762 and 763 in FIGS. 7A and 7B. Also,desiccant and heat transfer fluid flow rates can be set by pumps 609,653, and 655 in FIG. 6 and 753, 755, 709 and 743 in FIGS. 7A and 7B withmore flow through the regenerator and less through the conditionerincreasing concentration in cooling mode. Airflow rates are driven byfans 602 and 642 and damper 660 in FIG. 6 and 702, 742 and damper 760 inFIGS. 7A and 7B, with concentrations becoming lower as more air isdriven through coils 722, 733. Water addition at 652 and 752, 758 is themost direct way of reducing the concentration of the liquid desiccant.It also allows maintaining the concentration of liquid desiccant over awide range of conditions. Since the size of the tank is driven by theratio of the highest and lowest concentration used in the system,greater control over the concentration reduces the size required for thetank. Leading to a direct trade off in size and weight between liquiddesiccant storage and concentration management components.

FIG. 6 shows how the additional coils 622 and 671 are on the refrigerantcircuit. Heating and cooling mode are also realized by switching theevaporator and condenser 620 and 614 with four way switch 617. Thisresults in a complex refrigerant circuit with accumulator 615 b and 618b as well as a three way adjustable valve 618 and 619. Three expansionvalves are shown at 624, 638 and 639. Airflows can be adjusted with fans602 and 642 as well as the damper for 646B and a fan for airflow 672.Direct dilution 652 of liquid desiccant through module 657 can controlconcentration by controlling the flow rate at 652. Direct addition isalso possible in tanks. Using outside air or exhaust air for 641 and646B will significantly impact results. Using available exhaust air topreprocess 601 through energy recovery can improve efficiency. As doespost processing of air 606 prior to it entering the space e.g. throughan indirect evaporative cooler. All these options have been described inprior art. Several combinations give full flexibility of achievingtarget conditions for a full range of outside air conditions. Optimizingcontrols requires new approaches that use the dehumidificationcapability of the system and its capacity to achieve target conditionsin a single step in conditioner 603.

The main challenge for the system shown in FIG. 6 is controlling thequality of refrigerant in the system and thus system performance over afull range of conditions.

FIG. 7A shows a simplification of the refrigerant circuit by eliminatingcoil 671 in FIG. 6 and using instead 722 as a dual fluid coil with waterand refrigerant. The water circuit is set up to allow 722 to fulfill thefunction of 671 but using water rather than refrigerant. This avoids theneed for a parallel refrigerant circuit on the condenser side, which canbe critical in a reversible system. The detailed operations aredescribed in U.S. Pat. Ser. No. 10/024,558.

FIG. 7B shows a comparable solution with a much simplified heat pumpsystem with two refrigerant to heat transfer fluid heat exchanger 714and 720. Four way switch reverses the desiccant flow. This type of heatpump is well understood and has few challenges. The functions ofhumidity control coils 672 and 622 in FIG. 6 are now replaced by 733 incombination with damper 760. A properly sized coil 733 can be used inheating and cooling mode, either as a condenser coil to reject heat from720, which reduces the energy available for regeneration in 748 and thusreduces the concentration of liquid desiccant 752 coming out of theregenerator 748. A lower concentration of liquid desiccant will lead toa higher RH of air 706 leaving conditioner 703, thus shifting the airsupply towards more sensible and less latent cooling. This “heat dump”function of coil 733 is critical when conditions shift from hot andhumid to hot and dry. During cool and humid condition coil 733 is usedas an additional evaporator side coil, providing an additional load tothe compressor 715, which provides additional energy for regeneration to748, increasing the concentration of the liquid desiccant 752. This modeis shown in FIG. 7B with switches 765, 762, 763 and 764 set such thatconditioner 703 and coil 733 run in parallel. By diverting cooling fluidfrom conditioner 703 while increasing the concentration of LD 707entering the conditioner 703, the air 706 is dehumidified while the DBtemperature will start to rise as the flow of heat transfer fluid 704goes to zero and all heat transfer fluid is diverted by valve 765 to766.

For a system that needs to be able to deal with a very broad range ofconditions a simplified refrigerant circuit would be preferred with moreof the adjustment in the system configuration being done on the heattransfer fluid side. That does involve an efficiency loss driven by theefficiency of the LCE 714 and the LCC 720. But it eliminates the needfor multiple refrigerant switches, multiple expansion valves and theircontrols and for additional design to balance refrigerant and oil withreceivers and accumulators.

Managing the various coils on the refrigerant side of the system becomesmore difficult as the system gets more complex. A multi zone system isshown in FIG. 8. To cool spaces 810 with desiccant conditioners 821 inlow ceiling space 816 using a mixture of return air 817 and outside air809 from DOAS to supply air 818 to the space. The units 808 get coolingwater from chiller 814 through piping 812 and 813. Liquid desiccant isregenerated in 801 using the condenser heat of chiller 814. Theregenerator 801 can also be positioned close to available exhaust air tofurther improve the effectiveness of regeneration. The concentratedliquid desiccant is supplied via 802 and 805 to units 808 and 806. Andreturns via 803 and 804. When return air 817 is dry and the buildinglatent load is fully covered by deeply drying air 809 by outside airunit 806, than units 808 can be sensible only cooling solutions,including highly efficient sensible solutions like chilled beams.Controlling such a system with a VRF \poses challenges in managing therefrigerant. Instead a chiller solution in combination of liquiddesiccants heat exchangers and with heat transfer/water connections ismore efficient and can be easier to install, maintain and operate.

FIG. 9 shows a different solution for residential or small commercialsplit system. Flexibility in locating various components of the systemcan simplify the system and improve performance depending on thebuilding and its requirements. The configuration shown has multipleindoor units 903 inside the conditioned space. The conditioned spacescan be on different floors in the building and at different distancesfrom the regenerator unit. Managing the flows of heat transfer fluid 904and liquid desiccant 907 can be simplified by strategically locatingtanks 910, 954, desiccant pumps 909, 954, 953, heat exchanger 956, andwater pumps 913/944. In general the refrigerant system with heatexchangers 914, 920 and chiller 915 as well as the regenerator 948 andfan 947 the various air cooled coils like 922 will be located outside,however tanks and pumps as well as other features like the wateraddition module 0957 can be located in appropriate technical spaces inthe home, where water and/or drains and/or power is available, wheremaintenance can be done easily and where there is sufficient space fortanks and components. Noise from the pumps can also be a consideration.

The heat pump system as shown in FIG. 9 is relatively complex. Inpractice, simpler versions of the refrigerant system can be used.

Alternatively the air coils 922 and 671 can be connected indirectly tothe compressor via the first and second refrigerant to heat transferfluid coils 914 and 920. This significantly simplifies the refrigerantcircuit but requires additional valves in the heat transfer fluidsystem.

For example, the task of controlling the system becomes complex iftarget conditions and loads of the conditioned spaces differsignificantly. Some of the potential issues include:

Spaces with different input conditions of outside and return air to theconditioner. Some spaces may have exhaust air for regeneration othersmay not. Also the temperature and humidity of return air from the spacesmay differ and of course the proportion of outside air required for thespace, which often depends on occupancy and potential requirements forover pressurization.

Different loads in the space including high humidity loads from plants,pools, kitchens and people and high sensible loads from outside walls inolder buildings, lights, equipment etc.

Different targets, e.g., in stores high humidity is desirable ingreen/veggie sections and low humidity is desirable in refrigerantsections. As a result the required load per cfm and the requiredconcentration of liquid desiccant for matching user requirements coulddiffer significantly.

Liquid desiccant control systems need to be able to address this byadjusting the water temperature and the liquid desiccant concentrationsupplied to a specific space. This may require a more complex tanksystem that allows the regenerator to adjust the concentration of theliquid desiccant by using different airflows and a mix of outside andexhaust air. Another option for creating multiple concentrations ofliquid desiccant is to vary the temperatures and flows rates of heattransfer fluids supplied to the regenerator and the air cooled coilsdiscussed above.

FIG. 10A shows how multiple conditioners C1 through Cn (1009 a, 1010 a,1011A) provide different conditions air (1009 through 1011) to spaces1-N (1009 c, 1010 c and 1011 c). Water flows 1003 through 1008 vary thesupply of heat transfer fluid to the conditioners from evaporator coil1001. The condenser coil 1002 is connected via heat transfer fluid flows1020 through 1025 to conditions R1 through Rn 1026 a, 1028 a 1030 a.Less exhaust air and higher overall airflows 1026, 1028 and 1030 willdecrease the concentration of the regenerated liquid desiccant which canthan stored in one or more tanks shown as D1 b, D2 b and DnB. A lowerwater temperature provided to the regenerator further reduces theconcentration of the liquid desiccant by adjusting airflows 1025, 1027and 1029 to air cooled coils SC1, SCV2 through SCn. Obviously such acomplex system requires an appropriate building and system controlsystem that uses these variables to ensure that target conditions inspaces or zones 1-n can be maintained with minimal effort by compressor1000.

Referring to FIG. 11, direct dilution of liquid desiccant gives the mostdirect independent control of humidity and temperature supplied to thespace to be air conditioned, while optimizing the efficiency ofrefrigerant system 1199. Direct dilution is especially important in hotand dry conditions when over drying of the air is not desirable and/orwhere the desiccant could be diluted until it crystalizes. It also helpsmaintain temperatures in the regenerator 1198 within the operationalboundaries of the materials used in the regenerator. Direct dilution ofthe liquid desiccant can be done in a number of locations as shown inFIG. 11, including in tanks 1112/1154, in the lines to and from theconditioner 1157/1158 and as an integral part of heat exchanger 1156. Asdisclosed in U.S. Pat. No. 9,308,490, dilution of desiccant can be donewith a vapor transfer membrane unit 1100 or with a forward osmosismembrane unit, using feed streams that can use either potable water orwater with a lower ionic content than the LiCl solution. This includesseawater. The vapor transition modules 1157 with feed stream 1158 usethe high ionic content of liquid desiccant to drive the vapor transfer.The rate of water transfer is driven by the temperature of the liquiddesiccant e.g. liquid desiccant coming from the regenerator. It istherefore energy efficient and avoids any addition of minerals or lossof liquid desiccant. Alternatively, a forward osmosis can be used toremove minerals in a similar unit. Alternatively, demineralized watercan be added directly to tank 1110. Control of the volume of water addedto the liquid desiccant can be done directly by controlling the tanklevel or by varying the feed flows to the water addition membranemodules.

Direct dilution ensures that the RH level of supply air 1106 cannot fallbelow a minimum level RH_(min) which can be calculated from the LDconcentration LD % by a formula RH min=(100%−V×LD %)+effectivenessfactor), where V is a factor driven by the vapor pressure of the liquiddesiccant For LiCl V is about 2. The effectiveness factor is driven bythe sensible and latent effectiveness of the panel and tends to bebetween 5-10% on average for cooling LD at a concentration of 25% willresult in conditioned air at an RH of 55-60%. Minimum RH levels arecritical for a wide range of applications. Optimal living and workingconditions tend to have an RH between 40 to 70%. Maintaining aconcentration of 20-30% ensures that those humidity conditions arealways met. These results are based on extensive modelling of liquiddesiccant systems over a broad range of conditions as well as tests ofliquid desiccant panels.

An alternative desiccant dilution method is shown in 1100. Placing adirect evaporative pad or cooler in the incoming air stream of theregenerator or the conditioner also dilutes the liquid desiccant. Anozzle that creates a fine mist in the incoming air stream that quicklyvaporizes has the same effect. During very dry conditions outside airconditions 1101 a conditioner 1103 using fan 1102 can supply cool air1106 with a humidity at or just below a target DP. In this situation theconditioner 1103 can actually desorbs humidity from the liquiddesiccant, increasing rather than decreasing the concentration andpartially cooling the air. Evaporator coil 1100 in airstream 1146humidifies and cools to a high DP. And a low temperature, TheRegenerator 1148 will therefore absorb rather than desorb water vapor,diluting the liquid desiccant. In such extreme conditions theconditioner 1199 has a significantly lower load from evaporator 1114which only has to provide the remaining sensible cooling to achievetarget conditions. Therefore the heat load from condenser 1144 is lowwhich results in a cool regenerator 1148 that absorbs water vapor,dehumidifying air 1146 to 1149 thus diluting the concentrated liquiddesiccant from heat exchanger 1156 and pump 1153. Positioning tanevaporator pad 1100 in airstream 1101 the same effect, but exposes theairstream to the conditioned space 1106 to the evaporator pad, whileregenerator air 1149 is exhausted. The overall effect of humidifying theregenerator air has the same effect as directly diluting the liquiddesiccant. Instead of adding water to the desiccant through a vapormodule, the water is added indirectly via the air. The advantage of thisapproach is that evaporative pads are cheap and well understood. Thewater management including managing water quality and mineral contentwith appropriate bleed streams will differ depending on location andwater conditions. Suppliers of evaporative coolers are familiar with thewater management issues. The cost of pads is currently lower than thatof vapor transition modules. From a control perspective these twomechanisms require similar solutions.

Controlling water addition in liquid desiccant systems can be done in anumber of ways described below: by increasing the feed stream eithercyclically or through a variable speed pump, maintaining direct tanklevel control with a level sensor, adjusting flow rates of feed water tothe evaporator etc.

Often solid desiccant wheels are used to recover energy from an exhaustair stream. The same can be done with a “passive” liquid desiccantsystems. FIG. 12 discloses how the liquid desiccant panels 703 and 704can be used as an alternative for a full enthalpy desiccant wheel withcomparable efficiency. Instead of using two different technologies twosets of panels are used. The main set of panels 702 and 903 conditionsair 706 at 702 to supply air 101 and regenerates the desiccant 902 at903 using hot and cold water 704 and 708 returning it via 705 and 709 tothe evaporator and condenser of a separate chiller. The liquid desiccant714 used to condition the air is pumped from tank 712 through the heatexchanger 718 to 702. The diluted liquid desiccant 902 is pumped by 901to the regenerator 903 where the condenser heat 708 and the outside air102 regenerate the liquid desiccant with humid air 707 leaving the unit.

A second set of panels does not use a compressor system or an externalsource of heat or cold. Instead it preconditions the incoming air 706,with water 801 and desiccant 717 which is pumped with 716 from tank 715.Panels 704 uses dry and cool exhaust air 102 to regenerate the diluteddesiccant with the dry exhaust air 102, Water 802 has been warmed up bythe absorption in 703 and is cooled by the low temperature of exhaustair 102. The desiccant regenerated in 903 uses the standard componentsincluding a liquid desiccant tank 712, a heat exchanger 718, and pumps713 and 901. The tank allows for different concentrations of liquiddesiccant as the exhaust air volume and outside air temperature andhumidity change.

The liquid desiccant ERV in FIG. 12 uses additional panels in 703 and704 for energy recovery, but since the humidity and latent loads for thepanels 702 are significantly lowered by the energy recover process,fewer panels can be used for conditioner 702 and/or regenerator 903,resulting in cost effective solutions.

The prior art shows the following fundamental tools to manage humidityand temperature independently in the liquid desiccant systems describedabove.

-   -   1. Total cooling or heating is driven by the capacity of the        compressor system.    -   2. The liquid desiccant panels will use the available cooling        capacity to condition a combination of outside air, return air        or air pre-conditioned by an energy recovery device or an        evaporator unit. Higher airflows and/or higher heat transfer        fluid flows through the regenerator panels can increase the        ratio of sensible versus total cooling or the sensible heat        ratio (SHR). Low liquid desiccant flows can further increase the        SHR.    -   3. Adding an air cooled coil to the condenser side of the        compressor in parallel or in series with the regenerator on the        airside enables additional sensible cooling.    -   4. Adding an air cooled coil to the evaporator side of the        compressor that processes outside air or air from the        regenerator provides additional cooling power for deep        dehumidification in the conditioner increases latent cooling,        until latent cooling is larger than the total cooling capacity        resulting in heating of the air If the air cooled coil provides        all the load to the regenerator, the conditioner will dehumidify        adiabatically, resulting in a negative SHR.    -   5. Adding a sensible coil before the conditioner on the        evaporator side of the compressor and after the conditioner on        the airside maximizes sensible capacity.    -   6. Desiccant dilution allows more sensible cooling and less        dehumidification or even net humidification of supply air        Significantly increase the relative humidity of the supply air,        while reducing the temperature resulting in an SHR>1    -   7. Maintaining a minimum level of liquid desiccant with        demineralized water maintains a minimum RH level, e.g. 30% with        LD of about 35% concentration. Supply conditions can exceed this        minimum RH level, based on input and ambient air conditions and        fluid flows through the coils.    -   8. Preconditioning the air with direct evaporation has an        identical effect to desiccant dilution, but uses existing        components.    -   9. Exhaust air can be used on the conditioner to reduce the        overall load, either sensible (Plate HX) or sensible and latent        (full enthalpy HX incl. wheels, plates, and LD plates).    -   10. Exhaust air with a lower RH than ambient conditions can be        used at the regenerator to provide more efficient        dehumidification, since less condenser heat is required to        maintain a high concentration of liquid desiccant.

Air-cooled coils and liquid desiccant heat exchangers can be connectedeither directly via the heat transfer fluid system or indirectly via theliquid cooled refrigerant heat exchangers when the air cooled coilscondition air directly with refrigerant.

The above focusses on the use of fluid flow rates, sensible coils,desiccant dilution and exhaust air for independent management of latentand sensible cooling. It shall be clear to those skilled in the art thatsame options can be used for independent control of temperature andhumidity with the refrigerant system in heating mode.

FIG. 13A shows the main modes of modes of operation in the psychrometricchart. The starting and target conditions are typical for a DOAS system.The zones are defined relative to a target supply condition 1699.Systems with recirculation have similar requirements but with differenttarget conditions.

-   -   a. Cooling and deep dehumidification is a typical requirement        for many air conditioning systems in hot and humid climates (4).    -   b. Dehumidification with deep sensible cooling (3) requires        maintaining or increasing the relative humidity typically during        the heat of the day.    -   c. Warming the air while dehumidifying (zone 5, 6) is a        requirement with very humid but cool air typically in the early        morning and in spring or fall in moderate zones.    -   d. Humidification during cooling (1, 2) can be a benefit during        cooling of very dry air, e.g. desert air at a DP below 40 F with        a temperature of over 35 C. Typically this is a requirement in        areas where liquid desiccant systems are essential for monsoon        period (1604) but where part of the year or even day can be very        hot and dry.    -   e. In heating mode the main distinction is between zone 7 where        high humidity can lead to frost forming in heat pump mode and to        over humidification of the space and zone 8 where humidity is        too low and additional humidification is required.    -   f. Cool and dry air needs no conditioning and the system can        operate in economy mode. Other than traditional systems, liquid        desiccant solutions do have the option to humidify the air while        maintaining dry bulb conditions. In essence that is a heating        operation since the enthalpy of the air increases.

Economizer mode 1699 b is a set of conditions where further processingwill not occur and the air is supplied to the space as is.

In DOAS applications, the economizer zone 1699 b can include zones 1 and2 if the liquid desiccant unit is used as a highly efficientdehumidifier with an SHR that optimizes compressor performance. Thismaximizes overall system savings if highly efficient sensible-onlysystems are available for further cooling of the air, including but notlimited to geothermal cooling and indirect evaporative cooling.

FIG. 13A shows the key characteristics of the 8 zones in relation to thedesired target condition. In this disclosure, cooling refers to adecrease in DB and/or in WB condition and heating to an increase of DBand WB condition.

Dehumidification refers to a reduction in absolute humidity and/orrelative RH. While humidification refer to an increase in absolutehumidity.

FIG. 13B shows how in much of the eastern sea coast 1691 the coolingrequirements for outside air are mostly in zones 4, 5 and 6, while inthe southwest (Phoenix) 1692 conditions can differ from extremely hotand dry to hot and humid To cool and humid, covering the psychrometriczones 1 through 6. The eastern sea coast requires a relatively simplesystem with either only desiccant components or a single sensible coilon the evaporator side of the compressor and in series with theregenerator. Efficient operation in the south west requires some form ofdesiccant dilution/water addition.

FIG. 13C shows how the availability of exhaust air at condition 1693reduces the range of conditions to be managed. With target condition1699, exhaust air conditions 1693 and ambient condition 1695, 50% ERVwould reduce the conditions “seen” by the system to a narrower envelop1694. This allows for smaller sensible coils to be used for zone 2, 3and may make an evaporator side coil for 5 and 6 unnecessary.

FIG. 4 showed how temperature and humidity can be managed independentlyusing a number of coils. The following fundamental approaches can beused to manage humidity and temperature independently:

FIGS. 16A-16C show how fluid flow rates, sensible coils, desiccantdilution and exhaust air can be used for independent management oflatent and sensible cooling. The same options can also be used forindependent control of sensible and latent heating.

FIGS. 14A-14D summarize various control options and how they related totemperature and humidity control by describing how in each of the 8zones of FIG. 13, heat exchangers and water additions should be used toachieve target conditions. It also shows when using these coils is noteffective.

FIG. 14A shows a configuration with an air cooled coil connected to theevaporator side of the refrigerant system. The options with that coiloff shows where a base unit without any coils or water addition canoperate effectively.

FIG. 14B shows how dilution of the liquid desiccant can be used tofurther improve operations especially in zones 1, 2, 3, and 8. Thedegree of dilution in 3 and 8 depends on the humidity loads in thespace.

FIG. 14C shows how a sensible coil connected to the condenser site ofthe refrigerant system can improve operations in the 8 zones.

FIG. 14D shows how a single air cooled coil can be used to manage thezones.

FIGS. 14A-14D show how the various coils are used in the main modes withHX1 being the refrigerant to water heat exchanger 1701, HX2 therefrigerant to water HX 1702 and , HX3 the air cooled coil in serieswith the regenerator 1743. HX4 is the air-cooled coil 1770, which coolsthe condenser using outside air. HX3 and HX4 can be combined but thatrequires a damper between the regenerator 1048 and air cooled coil 1043to shift coil 1743 from exhaust air from the regenerator in mode 5/6 tooutside air in mode 2, 3. It also requires either a dual fluid coil or avalve system to switch the coil from running on the evaporator side tobeing part of the condenser side of the refrigerant system.

FIG. 14B shows in Table 14B for each of the 8 zones of FIG. 6 disclosinghow the air cooled coils and water addition can be used to achievetarget conditions. The system shown in FIG. 14 has fan 1702 supplyingair 1701 to conditioner 1703. The conditioner is cooled with heattransfer fluid 1704 from evaporator coil 1705 or HX1. Pump 1713circulates the heat transfer fluid 1704 from the conditioner back to therefrigerant to water HX 1705. The desiccant 1707 is pumped by 1709 fromtank 1710 to the conditioner 1703. The diluted desiccant 1708 returns tothe tanks after processing supply air 1706. The diluted liquid desiccant1711 is heated in heat exchanger 1756 before being supplied to theregenerator 1748 through pump 1753 as 1745. The concentrated liquiddesiccant 1752 is returned via an optional high concentration tank 1754by pump 1755 via heat exchanger 1756 where it is cooled by 1711 andreturns to the tank 1710 as 1712.

Humidity control is driven by the temperature of the water 1744 and theair as 1746 as well as the humidity of the air 1746 which is supplied byfan 1747 to regenerator 1748. Pump 1740 rotates heat transfer fluid 1744through refrigerant to water heat exchanger HX2 (1771) to regenerator1748. Lower flows of water 1744 or lower flows of air 1746 result in awarmer regenerator and thus in more concentrated liquid desiccant whichwill result in dryer supply air at 1706. Air cooled coil HX3 1743 coolsthe hot and humid air from regenerator 1748, providing a load to thecompressor 1799 to maintain a high temperature at the condenser 1771 inorder to concentrate the liquid desiccant 1745. This is used duringtimes where outside air 1701 is humid requiring dehumidification butcool, requiring sensible heating.

Heat dump HX4 (1770) runs in series or in parallel with liquid condensercoil 1771, reducing the heat available for regeneration and thusenabling the conditioner to cool 1708 more deeply without over-dryingit. HX4 can be directly connected to the condenser 9 or via therefrigerant to water HX2 1771 (link 8)

Water addition option 5 either with the membrane module 1757 or in adesiccant tank 171710 is critical in zones 1 and 2 of FIG. 14, and canmake a major contribution in zones 603 and 608. With water addition,liquid desiccant systems become among the most competitive solutions forany supply condition, with comparable performance better control oversupply conditions and using less water than existing evaporative systemEvaporator Coil HX6 at 1759 dilutes the desiccant indirectly byincreasing the humidity of the incoming airstream at the regeneratorresulting in significant absorption of vapor at regeneration 1748 whichallows conditioner 1703 to operate at least partially as an evaporativecooler.

FIGS. 14A-14D show how the various coils are used in the main modes withHX1 being the refrigerant to water heat exchanger 1701, HX2 therefrigerant to water HX 1702 and HX3 the air cooled coil in series or inparallel with the regenerator 1743. HX4 is the air cooled coil 1770which cools the condenser using outside air. HX5 and 6 describe thesetting for the direct water addition and air humidification.

FIG. 14D shows how HX3 and HX4 can be combined but that requires adamper 1710 between the regenerator 1048 and air cooled coil 1043 toshift coil 1743 from exhaust air from the regenerator in mode 5/6 tooutside air in mode 2, 3. It also requires either a dual fluid coil or avalve system to switch the coil HX3 (1743) from a direct or indirect (8)connection to the evaporator side of the compressor to a direct orindirect via HX2 connection 9 on the condenser side of the compressor

For those skilled in the art it will be clear that the solutions shownin FIGS. 14A-D are not intended to be limitative. For example in zone 6,the conditioner 1706 will operate adiabatically to minimize coolingunder conditions that already have an air enthalpy below that of thetarget. By accepting a somewhat dryer air condition, DB targetconditions can be realized without reversing the system. Coil 3 providesthe extra cooling load needed to maintain the high concentration ofliquid desiccant at 1752. Applying direct heat to heat transfer fluid1744 going into regenerator 1748 is potentially a simpler but lessefficient alternative. The solutions in the tables in FIG. 14 focus onmaximizing system efficiency in term of EER and MRE, however at a costin complexity and equipment costs. Zones 1 and 2 can also be served byusing (in)direct evaporative coolers after the conditioner. Similarlyzone 7 and 8 could be served by using gas, electric or waste heatsources when available. FIG. 14 does not seek to describe the impact ofenergy recovery from exhaust air supplied to the regenerator 1748 or theconditioner 1703 or to both. For those skilled in the art suchcombinations are clear.

FIG. 11 shows how the performance of the optimized liquid desiccantsystem outstrips that of existing dedicated outside air system by 50% ormore.

FIG. 15A shows typical target supply conditions for heating and coolingsystems on the Psychrometric chart in comparison to the standard coolingand heating comfort zones.

The target conditions are those supplied by the system to a space, whichenable the space to maintain comfortable conditions. Most airconditioners supply air 1903 at a cooler and dryer condition than thecomfort conditions 1902 in the space. This compensates for the sensibleand latent loads due to occupancy, heat infiltration and the load ofoutside air. As described in ASHRAE's DOAS design guide, direct outsideair systems can either supply room neutral conditions (1902) or ensurethat all latent needs of the space are met (1904), plus at least some ofthe sensible cooling compared to input air condition 1901, with theremainder 1905 being cooled by sensible heat only coils, cold beamsystem etc.

Target condition 1906 is in a heating mode with sensible and humidityloads, i.e. warmer than comfort conditions.

Comfort zone conditions will be realized in the conditioned space aftercooling and heating loads have been added at a given circulation rate.

The fan only or economizer mode is used during conditions at which noactive air processing is required and the unit operates in economizermode.

Economizer modes are a requirement for DOAS systems. ASHRAE recommendsthat the DOAS unit focusses on meeting the latent load requirements bybringing the air to a required DP condition. In general that conditionwill be lower than the target humidity, since the building will havelatent loads, which need to be compensated for. The economizer mode 1910of a DX system is limited to outside air that is already at target 1912DP and need no further dehumidification. Otherwise the DX system needsto overcool and then reheat the air. A solid desiccant system will drythe air at temperatures significantly higher than comfort levels,leaving the full load to be carried by the DX system that recirculatethe air.

Liquid desiccant air conditioning systems dehumidify and coolsimultaneously. With sufficient water addition/evaporative coolingcapacity they can reach any supply condition from any input condition1911. In that case the economizer mode is limited to the comfort zoneconditions. Without water addition LDAC systems maintain a maximum DP.

Typical recirculation systems have an economizer zone with a maximum butno minimum humidity level. This is a problem, since low humidity levelscan be harmful for health reasons, building quality and some kind ofequipment. LDAC units can be combined with water addition or evaporationto maintain both minimum and maximum humidity levels.

We will propose how such conditions can be managed simultaneously usingadaptive control systems without stability issues caused by conflictsbetween proportional—integral—derivative controller (PID) controllerloops.

FIG. 16 shows actual performance data for a liquid desiccant system withwater addition during a single day of operation. The water additionmaintained only a minimum desiccant level in a tank, 2001 shows actualoutside air conditions for a single day with 15 minutes intervals. 2002shows how a maximum humidity and temperature are maintained during athese rapidly changing outside air conditions using a very simpledesiccant dilution control that sets a maximum concentration levelthrough a minimum tank level control. 2002 shows the regenerator exhaustconditions that correspond to the supply conditions 2003. Theseconditions were extreme and can be found in only a few locations likethe red sea and the Arabian Gulf.

FIG. 17 shows the annual weather bin data for a number of major UScities. Apart from Phoenix all of these have RH levels above 30%. Theycorrespond mostly to zones 4, 5, 6 and 7 of FIG. 19.

FIG. 18 shows how the liquid desiccant system described here is able toachieve target conditions in a single step. This has major implicationfor the system control method which needs to control both temperatureand (relative) humidity. Standard DX cooling systems first cool the airuntil the required humidity level is reached and then reheat the air toachieve the desired sensible condition. That means that each subsystemrequires only a single control variable. For example in DOAS system theairflow requirements are driven by the ventilation needs of the space.The DP target is achieved by measuring air temperature of the air comingof the condensing coil until its equal to the target DP condition at2202. A second system is than used to reheat the air to the targetcondition 2203. Systems using desiccant wheels have a similar two stepapproach. First the air is dehydrated to the target DP 2204. Then in asecond step, it is cooled to the target condition. Most existingdesiccant systems including spray liquid desiccant systems heat the airup less than the solid desiccant wheel, however they still required asecond cooling coil to get air to the target DB condition. The liquiddesiccant system described above cools and dehumidifies in a single stepfrom start condition 2201 to target condition 2203. That requires acontrol that monitors both temperature and humidity and manages bothcompressor power and humidity controls.

FIG. 19A shows how a liquid desiccant system with a compressor isbalanced in an equilibrium situation with an outside air system. Thesame control principals apply to systems using a mixture of outside airand return or exhaust air. Given input condition 2301 at the conditionerand regenerator, the total enthalpy difference 2204 a between 2301 andthe supply air 2302 needs to be identical to the heat rejected 2304 bfrom the condenser to the air to regenerator exhaust air 2303 minus acorrection factor for the heat and friction losses of the compressor(about 20-30% depending on conditions, turn down etc.). This is shown inFIG. 19A for equal air flows at the conditioner and the regenerator.This is of course true for any compressor driven system. However,typical for a single step liquid desiccant system is that two othervariable have to be in balance as well to achieve a stable situation:

-   -   a. The humidity absorbed at the conditioner 2305 a needs to be        identical to the humidity evaporated at the regenerator 2305 b    -   b. The RH at the conditioner and the RH at the regenerator are        both in balance with LiCl at a concentration that is nearly the        same. Typically when the liquid desiccant has a concentration of        25% going into the conditioner it will come out at a        concentration between 23 and 24% and will return to 25% at the        regenerator. As a result the conditioners RH is likely to be 2        to 4% higher than the RH at the regen for the vapor pressures in        the air to be equal to the partial vapor pressure at the surface        of the liquid desiccant. The RH of the system is 1-2×        concentration ensuring the RH of the conditioner and regenerator        are closely correlated within 5%.

Such a system will always supply air at a concentration what below thatof the input condition, given that the energy available at theregenerator is always larger than the cooling power at the conditioner.Increasing the regenerator airflow will reduce the supply temperature at2306 b, which increases the RH of 2306B compared to 2304 b. This willreduce the concentration of the liquid desiccant. Since the totalcooling power remains the same, the WB condition will not change, butsupply conditions will shift to a cooler, more humid condition 2306 a.Increasing compressor power without changing the airflows will lead to asupply condition at a lower WB condition, but also to a lower RH at theregenerator and thus to deeper dehumidification at 2307 a.

FIG. 19B shows the effect of adding sensible coils at the condenser sideof the system. It allows for a greater delta enthalpy at the regenerator2304B for a given regen out RH condition at 2303 and removed humidity ofonly 2305 b. The lower rejection of humidity leads to a more humid,cooler supply condition 2302 for input condition 2301.

FIG. 19C shows the effects of three types of coils 2310. It shows thesupply condition with equal airflows over regen and conditioner.Regenerator air out is than 2311. Adding a heat dump coil 2304 inparallel with the regenerator reduces the regenerator temperature to2302 and increases the RH. This results in the lower concentration thatcauses supply conditions 2301. A heat dump fan 2303 in line with 2302 onthe air side increases condenser temperature. It creates the sameconditions 2301 but at a lower efficiency because the total liftincreases from 23 C (2330) to about 27 C (2331). The fan 2322 in linewith the regenerator, but connected to the evaporator has the net effectof increasing the condenser temperature and thus the concentration. Thiscreates a supply condition that is warmer and dryer. This shows howevaporator coil 2322, heatdump coil 2304 using outside air and the sameheat dump coil 2203 but using regen exhaust air can all be used tochange the supply conditions.

FIG. 19D shows that using how the moisture removal efficiency or MRE isoptimized when air is supplied at 2310. 2301 requires the unit to doadditional work as the heat dump fan increases the RH of theregeneration exhaust air to 2302. While total work increases theadditional work is done at a similar lift 2331 as both the condenser andregenerator temperatures decrease improving the overall efficiency(EER/COP). Using the heatdump fan to provide extra load for increaseddehumidification and a lower RH increases the total load withoutincreasing the amount of moisture removed. As a result, the moistureremoval efficiency drops. The lift 2331 is again comparable to 2330 asboth the supply temperature and the exhaust air temperature from theregenerator increase. Overall efficiency is reduced as the compressordoes about the same load, but the total cooling effect is lower. Inother words, a liquid desiccant unit has the highest MRE/moistureremoval efficiency when it is used without coils. Using the heatdumpcoil increases EER/COP but at a lower MRE. Using the advanceddehumidification coil reduces both. The advanced dehumidification coilprovides an advantage only when cool and humid conditions require acombination of latent heating and increased temperature. In that casethe reduction in enthalpy because of the increase in temperature can beconsidered useful work. This is especially true when the building hashigh internal latent loads because of occupancy, pools, plants etc. The920 standard recognizes this by penalizing a unit for supplying air at atemperature less than 70 F. As a result the ISMRE at 70 Fdb and 55 Fdpconditions improves significantly when the advanced dehumidificationcoil is used. This is especially important for the selection of unit inmaritime climates where cool and humid conditions requiringdehumidification together with heating is critical.

Ongoing discussion about the standard recognizes that in buildings withlow latent loads and high sensible loads, e.g., from lights, supplyingoutside air at temperatures below comfort conditions could be justified.When selecting dehumidifiers for hot and humid climates the MRE may bemore important than the ISMRE.

FIG. 19E show how the DB conditions can be managed independently fromthe target 55 DP using air cooled coils for each of the four 920conditions a (2304), b (2344), C (2354) and D 2364 Using an air cooledcoil in series with the regenerator allow the regenerator to take 920Dair and regenerate it to 2365 enabling the adiabatic dehumidification ofsupply of air 2364 to target condition 2361 at the same RH level. Thesame can be done for 920B 2354. 920D can supply condition 2311 withappropriate fluid flows in the regenerator. However, 920A will supplyair at 2310, reflecting the regenerator performance without heat dumpcoil.

FIG. 19E shows the benefit of using an the air-cooled evaporatoradvanced dehumidification coil in line with the regenerator. 2365 is theinput air condition of the coil, which cools it to 2366. This can bedone without a need to remove condensation from the coil, which can be asignificant benefit, especially if the unit is positioned inside abuilding with the outside air ducted in. It reduces maintenance andavoids any risk of water damage. The total lift of this combination islow. Alternatively, the coil could be used before the regenerator,providing it with dryer air 2371. The dryer air would allow theregenerator to regenerate at a lower temperature 2372. The lift over theregenerator is similar, suggesting a similar efficiency, however theadditional condense removal required nullifies one of the key advantagesof liquid desiccant systems that there is no need for condensemanagement.

FIG. 23F shows how air-cooled coils can be used to meet the supply airtarget of 70 F DB and f55 F DP (2310) for the 920 A, B and C conditions.For 920D, air is supplied at 70 F adiabatically. In other words, thedewpoint while the wetbulb condition is the same as 920D 2360. FIG. 19Fassumes that the advanced dehumidification coil uses outside air andthat it is not in line with the regenerator. The effect is thatadiabatic dehumidification is realized but with a higher lift then inFIG. 19E, and thus a lower efficiency. Also condense needs to bemanaged. But it does allow for a single air cooled coil to function asconditioner to running in parallel to the regenerator as is shown inFIG. 13. Damper 660 in FIG. 6 is a way to gain the efficiency benefitsof FIG. 19E while only using a single air cooled coil. This makes such asystem smaller, lighter and cheaper than a dual coil system, which isespecially important when cool and humid conditions are rare. It alsoallows the coil to be sized for the larger “heatdump” load, which makesadvanced dehumidification even more effective.

Still cooling outside air to 2359 and 2379 will lead to significantcondensation on the coil while providing the additional load needed toreconcentrate desiccant for 920 D and C. 920B has been shown to balanceto the 2310 supply condition without additional airflow over thesensible coil. 920A can match the 70/55 2310 condition by using the coilas a heat dump (2309) increasing the RH from 2310 in FIG. 19E to 2311 inFIG. 19F. It will be understood by those skilled in the art that thisfurther increases the flexibility by which humidity and temperature canbe managed independently using sensible coils.

FIG. 24 shows the impact of adding water to dilute the liquid desiccantas shown in FIG. 11, either through a vapor transfer unit as describedin U.S. Pat. No. 9,308,490, which is included by reference, throughdirect addition to a water tank or through evaporative pads as discussedin U.S. Patent Application No. 62/580270. In both cases the effect ismore diluted Liquid desiccant and thus a higher DP for the supplycondition. Diluted liquid desiccant can take very dry air just above thecrystallization point of liquid desiccant (2400) and supply air 2401 atthe same or even a higher DP supply target 2410. The regenerator will beevaporating strongly 2402 to exhaust air at 2411 maintaining and RH ofabout 50% RH e.g. using LiCl with a concentration of about 25%. Theconcentrations for other desiccants will differ.

Using evaporators 2403 prior to both the conditioner and the regenerator(2403) will provide both with input conditions 2413 at an RH of 75% orhigher. The conditioner 2404 and the regenerator 2405 will be able toachieve the same supply and exhaust conditions 2410 and 2411 assuming asimilar concentration of about 25% LiCl.

FIG. 20 shows how the system can be used to achieve humidity andtemperature conditions independently for a wide variety of conditions.In particular the very dry and hot condition 2400 which is cooled andhumidified 2401 with the regenerator conditions 2402 as described above.The same condition can be achieved using an evaporator pad 2403. Fromthe dry but less hot condition 2420, the same system can adiabaticallyhumidify and cool using either water dilution in combination with theconditioner or using a separate evaporator pad. While 2420 can beachieved using any direct evaporative cooler and 2410 in FIG. 20 usingan indirect evaporative cooler, only a liquid desiccant system canhandle both conditions while avoiding over humidification (2400) oroverly dry air (2420). For locations like phoenix such independentcontrol of humidity and temperature ensures that conditions can bemanaged year round with high efficiency and minimal water consumption.The same system can even be used to handle the cool and extremely humidcondition 2430, which can occur during early morning conditions in thePhoenix wet season. They can dry the air again to 50% RH 2431, byregenerating the liquid desiccant at 2432, albeit with the support of anair cooled coil providing additional load at 2433. Now often multiplesystems are needed and operators may need to switch in the early morningand late evening between different systems. The controls described belowseek to ensure that optimal conditions can be maintained around theclock. FIG. 20 showed how conditions in a single location can changethroughout the day from hot and humid to very hot and dry.

FIG. 21 shows the actual performance of an LDAC system for a range ofoutside air conditions and the target conditions 2500 the system wasable to maintain. It compares that with the performance of twoalternative systems. First an indirect evaporator cooler can be used tocondition 2501 but how it will leave conditions too humid in 2502, forexample. Conditions that are too humid. A direct evaporative coolercannot achieve conditions 2500 from the same outside air conditions.Instead it can either supply air that is much too hot at 2504 or cool to2503 but at too high humidity. For condition 2505 direct evaporativecooling is unsuitable. The Liquid desiccant system already demonstratedhow conditions 2500 can be achieved reliably for the conditions shown inFIG. 21, with water addition as shown in FIG. 20. With more advancedcontrol an even narrower range can be achieved.

FIG. 22 shows heating without frost forming by first dehumidifying andheating air adiabatically from 2601 to 2602 using the regenerator coiland then using the sensible coil to cool the dry air to 2603 with a DBcondition that is still higher than the DP. When conditions are very dryoutside as in 2604, water addition may be needed to achieve that targethumidity and temperature and temperature 2605. In that case theregenerator outside should be bypassed. At cool but humid temperatureslike 2610 where heating is required inside, the sensible coil can againbe used with regenerator turned down or off. This will result insignificant condensation at 2611 and to sensible heating only 2612 to2605. Like above these heating examples are not limitative, but show howa very broad range of conditions can be achieved with independentcontrol over humidity and temperature using a combination of a sensiblecoil, a valve system and/or desiccant dilution/water addition.

FIG. 23B shows how a DOAS system in the basic configuration of FIG. 4Acan still operate under a broad range of input conditions. However itdoes results in a somewhat broader “extended' set of target conditions2710. For the starting conditions 2711 the target T can be achieved andat performance will be significantly superior to existing DX and soliddesiccant wheel systems. For conditions 1712 the target condition Tcannot be achieved and the range of supply conditions 2710 will bebroader.

FIG. 23C shows how a DOAS system with the heat dump coil 429 in FIG. 4Bhas a narrower range of supply conditions 2710 and a broader set ofstarting conditions 2711 where those conditions can be maintained withsuperior performance to DX and solid desiccant wheel solutions.

FIG. 23C shows how a system with evaporative cooling and water additionhas superior performance at all conditions and can compete directly withwater cooled chillers and evaporative coolers in terms of efficiency andoverall system cost.

FIG. 24 shows how typically temperature and relative humidity aremeasured. A widely used sensor in HVAC systems is the T&RH sensor. Itmeasures dry bulb conditions and relative humidity. Controlling thesystem with DB and RH while possible, poses challenges since theconditions are not orthogonal, therefore RH conditions can be achievedat much too high DB conditions and DB can be achieved at very high orvery low RH conditions, which tends to make controls less stable.

FIG. 25 shows how in most HVAC systems T&RH measurements are translatedinto a measurement of temperature (DB) and of absolute humidity (DP,grains/lb. etc. They are orthogonal which makes control more intuitiveand direct. Simple bandwidth control than seeks to control supplyconditions by sequentially achieving DP (3001 a/b) and DB (3002 a/b)conditions. This is typically done by first achieving DP by eithercooling the DX coil until the DP condition is achieved or by using thedesiccant wheel to heat and dehumidify the air. A second step is neededto achieve DB supply conditions either by post cooling or post heatingthe process air using a separate coil.

FIG. 25 shows a typical control of HVAC system through a DB band 3002Ato 3002B and a DP target band between 3001 a and 3001B.

While traditional control sequences can also be used for such a systemusing bands of DB and DP conditions, the liquid desiccant also allowsfor an alternative approach show in FIG. 31. Here the DB and RH are usedto calculate the actual WB supply condition. The target WB condition3101 shows the total amount of cooling required from the startingcondition 3100 and shows the total compressor power required to get tothe target condition 3104. At a given setting for the humidity controlsthe compressor will cool to condition 3103. The humidity controls can beused to move the supply air from 3103 to 3104. Humidity controls caninclude:

-   -   Condenser linked sensible coil or heat dump to reduce desiccant        concentration and increase RH    -   Evaporator linked sensible coil or advanced dehumidification        coil to increase desiccant concentration and reduce RH    -   Water addition or evaporative cooling pads to directly dilute        the liquid desiccant and increase RH    -   Increase regenerator airflow to cool the condenser at a lower        temperature which reduces the concentration and increases RH    -   Reduce water flow in the regenerator to increase the heat        transfer fluid temperature and the condenser temperature which        increases concentration and reduces RH    -   Reducing heat transfer fluid flows at the conditioner to cool        the air adiabatically and increase the DB temperature and reduce        the sensible heat ratio

For those skilled in the art it will be clear that these examples arenot limitative. Other options have been discussed including a postcooling coil linked to the evaporator to shift increase SHR.

Heating can be done similarly by allowing independent control of thecompressor for overall heating effort and the humidity controls andfrost prevention controls using the sensible coils, fluid flows andwater addition/desiccant dilution.

FIG. 27 shows an alternative control strategy for DOAS systems with a DPtarget. That allows for a much broader range of conditions to beprovided by a DOAS without additional sensible coils or water additionfor conditions 3301, 3302 and 3303. A simple bandwidth control willwork. Regenerator Fluid can be kept high to maximize conditionerperformance. Only at very dry conditions 3304 and very wet condition3305 is it than desirable to use additional humidity control coils,first by turning down the fluid flows in the regenerator. Air cooledcoils could be used in addition to prevent crystallization 3304 andoverly humid air 3305.

FIG. 28 shows an alternative approach to measuring and controlling thedesiccant concentration by using the tank level 1401 rather than an RHdriven approach. If the system aims to maintain an RH of 50% aconcentration of about 25% needs to be maintained. That corresponds to agiven level 3401 in the tank 3400. The tank level can be measureddirectly through a level sensor 3402. As the concentration of LDincreases, the tank level drops. That can be done directly throughdilution if the tank level fall below target. A solenoid valve 3403triggered by a floater 3404 is a simple way of achieving this. As tanklevels increase other controls are triggered to maintain the targetlevel by increasing latent regeneration. As explained earlier additionalloads from a sensible coil can be used to generate more condenser heatto maintain liquid desiccant concentration levels. Alternatively lowerregenerator airflows and lower water flows will sensible coils need tobe used to concentrate the desiccant, e.g. through the advanceddehumidification coil regenerator water and airflows.

In addition to using tank levels, desiccant concentration can bemeasured in a variety of ways. A simple measure is the RH out of theregenerator or conditioner. For LiCl the percentage RH is approximately1-2*concentration LD. Direct sensors are another options however thecost reliability and accuracy of these sensors needs to be furtherdemonstrated. Multiple sensors in a system allow for more accuratesupply conditions, more effective predictive controls with fasterresponse times and system diagnosis e.g. detection of small leaks basedon readings of different sensors.

Liquid desiccant system with RH sensors in the regenerator exhaust,where the RH is used to calculate the LD desiccant concentration using aformula based on a combination of basic physics (equal vapor pressuresat equal RH at any temperature) and system know how (Latenteffectiveness of the panels). 1−X1(ld)*C−LD+Delta(system)=RH, whereX1(LD) is determined by the type of liquid desiccant used (ca 2-2.1 forLiCl) and Delta is driven by the system dimension (1-5%).

A Liquid desiccant system with a ventricle (narrowing of the pipe) canmeasure concentration based on pressure drop over the ventricle at agiven flow rate.

Specific weight can be used by using a floater with known specificweight in the LD, In that case the depth of the floaters determines theconcentration.

Diffraction uses the changing optical properties of the LD to measurethe diffraction of a known frequency laser beam to measure LDconcentration.

Electrical resistance uses changing electrical properties of the LD todetermine concentration. However for LiCl resistance plateaus in thecritical 20-30% range which requires this measure to be used incombination with other system properties, e.g. RH out.

Alternative adaptive control algorithms that focus on the specificproperties of the single step liquid desiccant cooling anddehumidification system include therefore the enthalpy/RH control, thetank level control. A potential problem with a single step adaptivecontrol system is that controlling multiple variables at the same timecan lead to conflicting PIC loops. This can lead to oscillations of thesystem control. A way to get to a single PID loop is to first determinewhether temperature or humidity level is further from target. Mostsystem measure Temperature and RH. The controller can covert that toenthalpy and absolute humidity conditions, i.e., WB and DP or RH. Thesystem PID loop can approach the target conditions for temperature andhumidity by determining which of the two variables is furthest oftarget. Whether that is the delta of actual versus target DB or WB orthe target versus actual DP or RH.

FIG. 29 shows how such an adaptive algorithm can work. Given arelatively high WB condition with an RH already close to target (3501),the compressor first adjust total enthalpy until the delta in RH (DP)becomes larger than the delta in the WB (DB) condition 3502. Acombination of water addition, heat dump/sensible coil fan, heattransfer flows or regenerator fan (in order of impact) can then be usedto adjust the humidity level until the RH/DP is closer to target thanthe WB 3503 and then the compressor is adjusted 3504.

FIG. 30A shows how humidity DP or RH target 3601 and DB target 3605 canbe used to calculate WB and RH targets 3620 and 3621. These are comparedto the calculated measured WB 3622 based on the measured DB 3603 and RH3605. The measured RH can be compared directly to the calculated RHtarget 3621 to be used by the controller to calculate the RH error 3624.The largest of the WB/enthalpy delta 3623 and the RH delta 3624 is usedto control the compressor through an adaptive PID loop. The PID loop itdrives compressor settings 3606, which in turn drives the regeneratorand conditioner flows 3611. A variable conditioner airflow willdetermine the relationship between the compressor settings and theregenerator and conditioner fluid flows 3611. The RH or DP delta 3624will drive the SHR setting 3630. This can be either the air cooledcondenser coil or heat dump fluid flows and possible damper setting 3612OR the water addition flows 3614 OR the advanced dehumidification(evaporator air cooled coil) flows 3613 OR evaporator coil settings(water flow/air. Multiple RH or DP and temperature settings can be usedto drive crystallization settings 3630.

A similar structure can be used for the adaptive control method shown inFIG. 29, where the largest error of WB/enthalpy and RH drives the PIDloop. Or through a band with control where a WB bandwidth is used tocontrol the compressor, while the RH is controls the humidity or SHRcontrols 3611 through 3615.

FIG. 36B shows a similar process flow, but now with the errors in DB andDP driving the system, where the largest error again drives the capacitycontrol and the humidity measure (DP) drives the SHR controls

Here the crystallization control 3631 is shown to be driven by thetarget DP and measured RH values for conditioner and regenerator.

FIG. 31 shows a simplified adaptive control logic based on actuallyimplemented systems with DP and DB target. The actual DP 3703 iscalculated from actual Temperature 3705 and the measured RH by thecontroller. Actual are compared to the DP target 3701 and the DB target3502. Resulting in a DP error 3704 and a DB error 3705. The largesterror controls the PID loop 3708, which controls compressor speed, andregenerator fluid flows. The humidity measure (DP) directly controls theair cooled coil airflows or water addition levels. Airflows are theprimary fluid control. Hot water and cold water flows are driven by thecompressor to match capacity. During very dry conditions or when thedesiccant is over diluted, desiccant flows to the regenerator orconditioner can be turned off allowing the system to recover.

Such an approach can use any combination of WB/RH/DB and DP to controlthe system. However WB/RH, and DB/RH and DP controls are particularlyimportant. The former can be used in a bandwidth controller. DB/RH isdirectly based on typical user settings. DP control is important whenthe Liquid desiccant unit is used as a dehumidifier with separatesensible cooling capacity.

FIG. 32A shows how such a system can work using a DB 3802 and DP setting3801. These two can be used to calculate a target RH 3806, which whencompared to actual RH in the supply air 3703 gives a RH error 3807.

The Actual DB condition in the supply airstream 3805 can be compared tothe target 3802 to calculate DB error 3808. The controller 3809identifies the larger of the two errors and uses that to drive theCompressor setting 3810. The regenerator fan speed 3810 is adjustedbased on the RH error to approach the DB condition as much as possiblethan the Actual RH has to be used in combination with the actual DB tocalculate the actual DP and thus the DP error. The largest error isagain the driver of the compressor through the PID. While the RH errorcan adjust the regenerator fan to get closer to the target humiditycondition. If the DB target is translated in an RH at the target DP,than DP/RH can be used, with RH driven by fluid controls for the variouscoils and the regenerator and the DP kept on target with the compressorspeed.

FIG. 32B shows a more complete view of these controls using DB and RHerror to control including protection against crystallization andalgorithms that use input versus target conditions to set the rightsystem configuration settings or system “value settings”. It also showshow Liquid desiccant dilution settings, heat transfer fluid flows andheat dump/sensible coil fan settings are alternatives to reduce theDP/RH error. User setting can be DP (3701), DB (705.

FIG. 32B shows some of the key control options that can be used tooptimize liquid desiccant performance depending on key considerationslike overall efficiency, response time, system complexity and cost. Itincludes a crystallization controls 3812, high water temperatureprotection 3813 to ensure that the system doesn't overheat, tank levelcontrols to ensure that the tanks don't overflow or dry out. Several ofthese can be used to adjust RH targets 3806 to maintain a single PIDloop control. Direct control for of the compressor speed is necessarywhen the safety controls come within the safety margin. Full controlthat will require strong humidity control capabilities such as liquiddesiccant dilution rates or heat dump fan (regenerator air cooled fan)settings.

The conditioner fan in can be variable or fixed but is set independentlyof the other variables within an allowed bandwidth. Especially inoutside air unit, conditioner airflow 3818 should not be used to controlconditions. This is of course different in recirculation units whereairflow is one of the controls of the PID controller.

Input conditions to the conditioner and regenerator can be compared totarget 3820 and can be used to accelerate the adaptive controls 3821.This can also be used as the main control esp. in outside air systems.The controller also has to automatically adjust valve settings 3821. Incombination with ambient condition forecast or supply requirementforecast the controller can be optimized to anticipate future demand andcapacity (Ambient air temperature and humidity) for dehumidification.

Controls for liquid desiccant systems are focused on humiditymanagement. Using temperature as the primary driver can be necessarywhere the unit is the only option. However in that case additionalsensible cooling capacity needs to be added. A coil in the supply airfrom the conditioner to the space heated by the condenser (hot gasreheat) is an effective alternative to the condenser coil and hascomparable efficiency. Both the advanced dehumidification coil and thehot gas reheat coil operate at low compressor lifts during cool andhumid conditions. Both provide additional load to the compressorrespectively for concentrating the liquid desiccant and for overcoolingair and then reheating it. The choice will be driven by the cpst of themore complex refrigerant systems with another hot gas coil and by thebenefits that the advanced dehumidification coils have in heating mode,in particular the frost free heating avoiding or significantly reducingdefrost cycles which is made possible by the additional coil.

FIG. 32C shows how latent 3850 and capacity 3851 control are driven byrespectively the error in the actual humidity measures RH or DP (3807)and the largest of the two errors 3806 *humidity and Temperature/energylevel of the air (DB/WB) which drives the compressor speed which inturns drives the hot and cold water flows and the regenerator fan,Predictive algorithms can be integrated in these algorithms improveoverall system efficiency in particular hourly, daily, weekly and annualinformation on weather and occupancy and load requirements.

Such continuous learning and improvement can be integrated in thesystem. Alternatively it requires monitoring of field systems tooptimize controls based on actual usage. It also requires an ability toupgrade software controls remotely. This is a critical requirement forliquid desiccant systems. Users are not familiar with the capabilitiesof these systems and are used to the limitations of the traditional twostep control systems. Remote monitoring and control will acceleratelearning from the user community and improve performance and acceptanceof the systems. Wireless connections enable high reliability usage ofthe system.

Remote monitoring of the input and output conditions as well as theactual settings of the system is also a primary driver of preventivemaintenance where deviations from historical performance indicatepotential problems with system components.

Increasingly manufacturers use their pool of customers to learn whatgives the best performance. An adaptive liquid desiccant system willcombine direct feedback from field units with app based feedback fromusers. This can use a combination of smart algorithms with “wisdom ofthe crowd” based selection among those algorithms. For this to be usefula variety of settings for the units is needed. For example, humiditycould be more of a problem when people are active or when they aretrying to sleep than when they sit and relax. That could influence howmuch humidity levels fluctuate and how fast the system needs to respond.There are many crowd based systems that allow gathering that kind ofinfo. Combining these systems with info on liquid desiccantair-conditioning solutions with a focus on humidity will lead to rapidacceptance of such systems.

Predictive controls shown in 38 C can take different forms.

Predictive controls based on the existing detailed system models whichcan use input conditions to set compressor and humidity controls toquickly move to target

Learning models that actively perturbs (within limits) unit settings tomeasure effect on efficiency and learns the combinations of operatingparameters that work best to minimize energy usage.

Predictive models are particularly important when there are large cyclesin humidity during the day, or predictable swings in requirements(weekends) or time of year. For example a model can how dehumidificationrequirements and sensible loads will vary over the course of a day.Early in the day, outside air conditions will be humid and cool,requiring highly concentrated desiccant. During the middle of the daylarge sensible loads produce the heat required to deeply concentrate theliquid desiccant. Using a sufficient volume of liquid desiccant allowsstorage of concentrated liquid desiccant that can be used during cooland humid nights. Similarly at the end of the day, where cooling anddehumidification requirements are likely to remain large until wellafter sunset, loads can be reduced by using solar heat or solar power tostore highly concentrated desiccant to reduce demand in the earlyevening where the “duck shaped” network power demand curves couldbenefit from shifting some of the load from early evening to lateafternoon.

Predictive models can use a combination historical data and last minutedata, similar to the pricing models of airlines. Airlines set pricesbased on day of week and date, but then adjust based on actual buyingbehavior. A liquid desiccant predictive model can similarly use day ofyear/and hourly data to predict settings and then adjust based on last24 hours and current data. For example this could allow a system to gointo special dehumidification mode in September if temperatures the daybefore were 70 F DP 60 F+, but stay in standard cooling mode in Julywhen that probably represents a rainy day, below seasonal averages.

Bandwidth control systems will show fluctuations between the upper andlower boundaries of the bandwidth. However given that the buildingchanges all the air only once every 10-30 minutes fast response is oftennot required. Bandwidth control is simpler and can work effectively witha liquid desiccant system by using a combination of WB and RH bandwidthand measurements to drive the controls. Any input settings (DB/DP/RH)can be used to calculate the WB and RH bandwidth.

Referring to FIG. 33, control models can be equipment and buildingbased. In building based systems, T&RH conditions of the space 4001 andairflow requirements are used to control the unit, rather than (only)the equipment supply conditions 4002 Return air conditions 4004 are agood proxy for 4001 assuming appropriate adjustments. The slowness ofchange in building conditions requires a slow response system to avoidfluctuations. In combination with ambient data 4003 and time of dayinformation a predictive system can make optimal use of changingconditions to minimize energy use. Using the liquid desiccant system asto store the dehumidification capacity of concentrated liquid desiccantwill significantly reduce energy use by reducing the need for using aircooled coils and their additional loads. In combination with wateraddition capacity the liquid desiccant system becomes highly efficientand responsive to expected changes in conditions. to use simplercontrols that allow humidity and temperature to fluctuate in a narrowband.

Liquid desiccant systems do need to control for crystallization. FIG.34A shows the conditions for crystallization for LICL. Similar diagramsexist for CaCl and other desiccants.

Avoiding crystallization requires that nowhere in the panels thetemperatures and humidities shown in FIG. 34A are achieved. The DP asfunction of temperature was calculated using the vapor pressure andsolubility equations of the liquid desiccant. The system was thancontrolled by that function plus an offset reflecting the uncertaintyabout the precise dewpoint and temperatures realized in the unit giveninput and output DP and temperature. The DP of both the input and outputconditions had to stay above the DP at the supply temperature. A similarequation was developed using RH as a function of Temperature which canbe used in systems that control RH and temperature. Other combinationsof T, RH, WB and DP can be realized using the same basic set ofequations. The vapor pressure and solubility equations for LiCl weredeveloped by among others Conde. FIG. 34 shows how the crystallizationboundary is determined by concentration and temperature. Crystallizationcan occur in part of the panel where lower temperatures or lower RHconditions push the LiCl over the crystallization boundary 4101. ForLiCl that zone rangers from 40% LiCl at 0 C to about 50% LiCl at 60 C(4102. Maintaining a minimum LiCl level in a storage tank correspondingto a concentration of about 35% prevents potential crystallization atany conditions.

FIG. 34B also shows how a typical east coast operating zone 4104 nevergets close to crystallization conditions in cooling mode. That foroperations in hot and dry climates like Phoenix 4105 desiccant dilutionthrough evaporation or direct dilution is 4106 essential. In heatingmode the chart suggest that an optimal LiCl concentration of about 25%to 35% is best to avoid frost forming on the outside unit, resulting inan RH of 35-45% for the heated air. The higher concentration isnecessary to maintain dewpoints below 55 F at supply temperatures ofover 30 C.

FIG. 35 describes a sequence using water temperatures, dry bulb anddewpoint conditions as well as desiccant tank levels to identify when acrystallization warning is required. Protection includes water additionto the tank or by diluting the desiccant, turning down the compressor toreduce water temperatures, change regenerator airflows to decreaseregenerator desorption. For those skilled in the art it is clear that avariety of controls and measures can be used depending on experiencewith the system, frequency of crystallization inducing conditions andperformance requirements. For systems without water addition operatingin hot and dry regions in the US, improving crystallization protectionis important to reduce safety margins. Predictive controls are used withdata from modelling and testing. Direct concentration measurement can beused, e.g. by measuring the pressure loss across a restriction. The tanklevel is another predictor of concentration. RH out at high temperaturesis a strong predictor of concentration, and will improve using systemmonitoring. Combinations are used to identify problems, e.g. flowrestrictions, leaks and subsystem performance. Data history can be usedon key performance metrics to identify reduced performance that may bedue to lack of maintenance i.e. filter pressure drop too high, thanfilters need to be cleaned.

FIG. 36 describes how desiccant concentration and temperature determineviscosity of the desiccant. The chart describes LiCl, similar data areavailable on other desiccants. For a typical liquid desiccant systemoperating in cooling mode, dynamic viscosity in the regenerator is 2-5×larger than in the conditioner. As a result the conditioner requireshigher pressure to maintain flows, or may have a lower flow rate thanthe regenerator. Modelled flow rates for different concentrations can becontrolled in a number of ways.

-   -   Use of fixed flow pumps to ensure that flow rates at conditioner        and regenerator are equalized.    -   Use of pressure controls through valves or air references to        maintain a higher pressure for more viscose desiccants.    -   Accept differences in flow rates on the conditioner and        regenerator, the tanks or valves can be used to balance the        regenerator and conditioner.

Having thus described several illustrative embodiments, it is to beappreciated that various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to form a part of thisdisclosure, and are intended to be within the spirit and scope of thisdisclosure. While some examples presented herein involve specificcombinations of functions or structural elements, it should beunderstood that those functions and elements may be combined in otherways according to the present disclosure to accomplish the same ordifferent objectives. In particular, acts, elements, and featuresdiscussed in connection with one embodiment are not intended to beexcluded from similar or other roles in other embodiments. Additionally,elements and components described herein may be further divided intoadditional components or joined together to form fewer components forperforming the same functions. Accordingly, the foregoing descriptionand attached drawings are by way of example only, and are not intendedto be limiting.

1. A method of operating a liquid desiccant air-conditioning system tomaintain target temperature and humidity level in a space, the liquiddesiccant air conditioning system comprising: a conditioner for treatinga first air stream flowing therethrough and provided to the space as asupplied air stream, said conditioner using a heat transfer fluid and aliquid desiccant to treat the first air stream; a device for measuringtemperature and a device for measuring humidity in the supplied airstream; a regenerator connected to the conditioner such that the liquiddesiccant can be circulated between the regenerator and the conditioner,the regenerator causing the liquid desiccant to desorb water vapor to asecond air stream or to absorb water vapor from the second air streamdepending on a selected mode of operation of the system; a refrigerantsystem; a first refrigerant-to-heat transfer fluid heat exchangerconnected to the conditioner and the refrigerant system for exchangingheat between the refrigerant heated or cooled by the refrigerant systemand the heat transfer fluid used in the conditioner; a secondrefrigerant-to-heat transfer fluid heat exchanger connected to theregenerator and the refrigerant system for exchanging heat between therefrigerant heated or cooled by the refrigerant system and the heattransfer fluid used in the regenerator; and a system controller forcontrolling operation of the system; wherein the method comprising thesteps of: (a) measuring the temperature and humidity level in thesupplied air stream; (b) comparing the temperature measured in (a) to atarget temperature to determining a temperature error, and comparing thehumidity level measured in (a) to a target humidity level to determine ahumidity error; (c) comparing the humidity error and the temperatureerror on a common scale to determine the greater error; (d) using thegreater error to drive the system controller to control operation of thesystem to reduce the greater error; (e) repeat (a) through (d) aplurality of times.
 2. The method of claim 1, wherein the temperaturemeasured in (a) is a dry bulb temperature and the humidity levelmeasured in (a) is a relative humidity level, and wherein the targettemperature is a dry bulb temperature and the humidity target is a dewpoint target; and the step (c) is based on errors in dry bulb and dewpoint.
 3. The method of claim 2, wherein when the system operates as adehumidifier, the system controller is operated to prevent the dew pointbased on measurements from being lower than the target dew point.
 4. Themethod of claim 2, wherein the system controller controls the setting ofa compressor in the refrigerant system to control the cooling capacityof the system, and the system controller controls the liquid desiccantand heat transfer fluid flow rates in the regenerator, wherein higherliquid desiccant and heat transfer fluid flow rates in the regeneratorincrease the sensible cooling rate of the supply air stream.
 5. Themethod of claim 4, wherein the liquid desiccant air conditioning systemfurther comprises a liquid desiccant dilution device, and the methodfurther comprises controlling the sensible cooling rate of the supplyair stream by diluting the liquid desiccant using the liquid desiccantdilution device.
 6. The method of claim 5, wherein the liquid desiccantdilution device is used to maintain minimum level of liquid desiccant ina liquid desiccant tank of the liquid desiccant air conditioning systemto maintain a minimum relative humidity level in the supply air stream.7. The method of claim 4, wherein the liquid desiccant air conditioningsystem further comprises an air-cooled coil associated with the secondrefrigerant-to-heat transfer fluid heat exchanger, the method furthercomprises increasing the sensible heat ratio by increasing fluid flowrates through the air-cooled coil.
 8. The method of claim 4, wherein theliquid desiccant air conditioning system further comprises an air-cooledcoil associated with the first refrigerant-to-heat transfer fluid heatexchanger, the method further comprises reducing the sensible heat ratioby increasing fluid flow rates through the air-cooled coil.
 9. Themethod of claim 1, wherein the temperature measured in (a) is a dry bulbtemperature and the humidity level measured in (a) is a relativehumidity level, and wherein the target temperature is a dry bulbtemperature and the humidity target is a dew point target; and the step(c) is based on errors in wet bulb and relative humidity.
 10. The methodof claim 9, wherein the wet bulb error controls the setting of acompressor in the refrigerant system to control the cooling capacity ofthe system, and the relative humidity error controls the liquiddesiccant and heat transfer fluid flow rates in the regenerator, whereinhigher liquid desiccant and heat transfer fluid flow rates in theregenerator increase the sensible cooling rate of the supply air stream.11. The method of claim 10, wherein the liquid desiccant airconditioning system further comprises a liquid desiccant dilutiondevice, and the method further comprises controlling the sensiblecooling rate of the supply air stream by diluting the liquid desiccantusing the liquid desiccant dilution device.
 12. The method of claim 11,wherein the liquid desiccant dilution device is used to maintain minimumlevel of liquid desiccant in a liquid desiccant tank of the liquiddesiccant air conditioning system to maintain a minimum relativehumidity level in the supply air stream.
 13. The method of claim 9,wherein the liquid desiccant air conditioning system further comprisesan air-cooled coil associated with the second refrigerant-to-heattransfer fluid heat exchanger, the method further comprises increasingthe sensible heat ratio by increasing fluid flow rates through theair-cooled coil.
 14. The method of claim 9, wherein the liquid desiccantair conditioning system further comprises an air-cooled coil associatedwith the first refrigerant-to-heat transfer fluid heat exchanger, themethod further comprises reducing the sensible heat ratio by increasingfluid flow rates through the air-cooled coil.